The present invention relates to a magneto-optical recording medium, and particularly a magneto-optical recording medium of high resolution.
In a magneto-optical recording and reproduction method, local heating by irradiation with laser light is carried out to form information record pits, or bubble domains, and the recorded information is read through a magneto-optical interaction, i.e. the Kerr effect or the Faraday effect. When this method is adopted, increasing the density of magneto-optical recording may be accomplished by reducing the size of the record pits. In such a case, a problem arises as to the resolution (resolving power) in reproduction. The resolution is determined by the laser wavelength .lambda. and the numerical aperture N.A. of an objective lens which are used for reproduction.
A conventional magneto-optical recording and reproduction system will now be explained with reference to FIG. 1. FIG. 1A shows a schematic top plan of a record pattern, in which a magneto-optical recording medium 3 such as a magneto-optical disk has record pits 4 (hatched areas) formed, for instance, according to two-valued information "0" and "1", in a land portion 2 bounded on both sides by grooves 1, for example. The method of reproduction in use of such a magneto-optical recording medium will be explained, with reference to the case where the beam spot of reading laser light incident on the magneto-optical recording medium 3 is a circular spot, as is denoted by reference sign 5. When the pit interval is so selected that only one record pit 4 can be present in a single beam spot 5, as shown in FIG. 1A, each area irradiated with the reading laser beam will exhibit either of two kinds of status. Namely, the irradiated area has either one record pit 4 or no record pit in the beam spot 5, as respectively shown in FIG. 1B or 1C. Where the record pits 4 are arranged at regular intervals, therefore, an output waveform obtained from the recording medium 3 may be one that is alternatingly positive and negative with respect to a reference level 0; for instance, the output waveform may be a sinusoidal one, as shown in FIG. 1D.
On the other hand, where the record pits 4 are arranged in high density as shown in a schematic top plan of a record pattern in FIG. 2A, a plurality of record pits 4 come under the beam spot 5 simultaneously. Referring to three successive record pits 4a, 4b and 4c, for instance, the reproduction output obtained when the adjacent record pits 4a and 4b are located in a single beam spot 5 does not differ from the reproduction output obtained when the record pits 4b and 4c are located in the beam spot 5, as shown in FIGS. 2B and 2C. Therefore, the reproduction output waveform will be, for example, rectilinear as shown in FIG. 2D, and the reproduction outputs in the above two situations cannot be distinguished from each other.
Thus, in the magneto-optical recording and reproduction system generally used in the prior art, the record pits 4 formed on the magneto-optical recording medium 3 are kept as they are during reading of recorded information. Therefore, even if high-density recording, i.e. formation of record pits in a high density, is accomplished, a high S/N (C/N) cannot be obtained due to limitations as to resolution in reproduction. In short, satisfactory high density recording and reproduction cannot be achieved according to the conventional magneto-optical recording and reproduction system.
In order to solve the S/N (C/N) problem, it is necessary to improve the resolution (resolving power) in reproduction, and there arises another problem that the laser wavelength .lambda., the numerical aperture N.A. of the objective lens, etc. impose restrictions on the resolving power. As a countermeasure against these problems, the present applicant has previously proposed a superhigh-resolution (superhigh resolving power) magneto-optical system for recording and reproduction (the system will be hereinafter referred to as "MSR") (Refer to, for example, Unexamined Japanese Patent Publication HEI 1-225685 entitled "Magneto-optical recording and reproduction process" and Unexamined Japanese Patent Publication HEI 1-229395 entitled "Signal reproduction process for magneto-optical recording media", incorporated herein).
An explanation will now be given of the MSR. In the MSR, a temperature distribution produced by relative movement of a magneto-optical recording medium and a reproducing beam spot 5 is utilized so as to ensure that record pits 4 on the magneto-optical recording medium will, in reproduction, be generated only in a predetermined temperature region, resulting in a higher resolution in reproduction.
Examples of the MSR system include so-called relief type and extinction type reproduction systems.
First, the relief type MSR system will be explained with reference to FIG. 3. FIG. 3A is a schematic top plan showing a record pattern on a magneto-optical recording medium 10, and FIG. 3B is a schematic sectional view showing a magnetization mode of the same. As shown in FIG. 3A, the magneto-optical recording medium 10 is moved in the direction of arrow D, relative to a laser beam spot 5. As shown in FIG. 3B by way of example, the magneto-optical recording medium 10 such as a magneto-optical disk used here comprises at least a reproduction layer 11 composed of a perpendicular magnetization film and a recording layer 13; preferably, the recording medium 10 further comprises an intermediate layer 12 interposed between the layers 11 and 13. Arrows in the layers 11, 12 and 13 each represent schematically the orientation of a magnetic moment. In the example shown, the downward orientation corresponds to an initialized state. Information record pits 4 in the form of magnetic domains are formed at least in the recording layer 13 by upward magnetization, as viewed in the figure.
In a reproduction mode of the magneto-optical recording medium 10, first an initializing magnetic field Hi is applied externally, whereby the reproduction layer 11 is initialized by being magnetized in a downward orientation as seen in FIG. 3B. Namely, the record pits 4 in the reproduction layer 11 disappear. In this case, in the areas where the record pits 4 are present, the magnetization directions of the reproduction layer 11 and the recording layer 13 are kept opposite to each other by a domain wall produced at the intermediate layer 12, so that the record pits 4 are left as latent record pits 41.
On the other hand, a reproducing magnetic field Hr in the opposite direction to the initializing field Hi is applied to the magneto-optical recording medium 10, at least in a reproduction area thereof. As the medium 10 in this condition is moved, the region including the latent record pits 41 initialized as above comes to fall under the beam spot 5. Then, when the portion heated by irradiation with the laser beam is moved to the front end side of the beam spot 5, i.e. leftward in FIG. 1, a substantially high-temperature region 14 encircled with broken line (a) and hatched in the figure is generated on the front end side of the spot 5. In the region 14, the domain wall at the intermediate layer 12 is lost, and the magnetization of the recording layer 13 is transferred to the reproduction layer 11 by exchange force. As a result, the latent record pits 41 present in the recording layer 13 are duplicated in relief in the reproduction layer 11, as reproducible record pits 4.
Therefore, the record pits 4 can be read by detecting the rotation of the polarization plane caused at the beam spot 5 by the Kerr effect or the Faraday effect, according to the magnetization direction of the reproduction layer 11. Furthermore, in a low-temperature region 15 other than the high-temperature region 14 in the beam spot 5, the latent record pit 41 is not duplicated in relief in the reproduction layer 11. In the beam spot 5, consequently, the readable record pit 4 is present only in the hatched, narrow, high-temperature region 14. As a result, even in the case of such a recording density that a plurality of record pits 4 come under the beam spot 5 at the same time, namely, even in a magneto-optical recording medium 10 for high-density recording, it is possible to read only a single record pit 4, and hence to achieve high-resolution reproduction.
In order to perform such reproduction, the initializing magnetic field Hi, reproducing magnetic field Hr as well as the coercive force, thickness, magnetization and domain wall energy of each magnetic layer, etc. are selected according to the temperatures of the high-temperature region 14 and low-temperature region 15 in the beam spot 5. Namely, where the reproduction layer 11 and the recording layer 13 have coercive forces H.sub.C1 and H.sub.C3, thicknesses h.sub.1 and h.sub.3, and saturation magnetization (M.sub.S) values M.sub.S1 and M.sub.S3, respectively, the condition for initializing the reproduction layer 11 only is given by the following Equation 1: EQU Hi&gt;H.sub.C1 +.sigma..sub.w2 /2M.sub.S1 h.sub.1 (Equation 1)
where .sigma..sub.w2 is the interfacial domain wall energy between the reproduction layer 11 and the recording layer 13.
Also, the condition for the information recorded in the recording layer 13 to be maintained under the magnetic field is given by Equation 2: EQU Hi&lt;H.sub.c3 =.sigma..sub.w2 /2M.sub.s3 h.sub.3 (Equation 2)
Further, in order that the domain wall at the intermediate layer 12 between the reproduction layer 11 and the recording layer 13 may be maintained after passage under the initializing magnetic field Hi, the condition expressed by the following Equation 3 is required. EQU H.sub.c1 &gt;.sigma..sub.w2 2M.sub.s1 h.sub.1 (Equation 3)
As for the temperature T.sub.H selected to be in the high-temperature region 14, the condition expressed by the following Equation 4 should be satisfied. EQU H.sub.c1 -.sigma..sub.w2 /2M.sub.s1 h.sub.1 &lt;H.sub.r &lt;H.sub.c1 +.sigma..sub.w2 /2M.sub.s1 h.sub.1 (Equation 4)
By application of a reproducing magnetic field Hr which fulfills the condition of Equation 4, the magnetization of the latent record pits 4 in the recording layer 13 can be transferred to the reproduction layer 11, that is it is duplicated in relief as record pits 41 in the reproduction layer 11, and only in the area where the domain wall formed by the intermediate layer 12 is present.
Although the magnetic recording medium 10 used for the MSR system above has been explained with reference to a three-layer structure comprising the reproduction layer 11, intermediate layer 12 and recording layer 13, a four-layer structure may also be adopted in which a reproduction sub-layer 31 is provided on the side of the intermediate layer 12 with respect to the reproduction layer 11, as illustrated by a schematic sectional view thereof in FIG. 4.
The reproduction sub-layer 31 functions in aid of the characteristics of the reproduction layer 11, and compensates for the coercive force of the reproduction layer 11 at room temperature. The presence of the reproduction sub-layer 31 ensures that the magnetization of the reproduction layer 11 aligned by the initializing magnetic field Hi can exist in stable fashion in the presence of the domain wall, and the coercive force of the reproduction layer 11 is reduced drastically in the vicinity of the reproduction temperature. Thus, the domain wall is confined in the intermediate layer 12 and is permitted to spread into the reproduction sub-layer 31 to finally reverse the magnetization of the reproduction layer 11. This is accompanied by disappearance of the domain wall. As a result, the record pits in the recording layer 13 can be duplicated in relief in the reproduction layer 11 in an improved manner.
When the four-layer structure including the reproduction sub-layer 31 is adopted, the coercive force H.sub.c1 of the reproduction layer 11 is replaced by H.sub.CA defined by the following Equation 5, and .sigma..sub.w2 /M.sub.S1 h.sub.1 by .sigma..sub.w2 /(M.sub.s1 h.sub.1 +M.sub.s1s h.sub.1s). EQU H.sub.CA =(M.sub.s1 h.sub.1 H.sub.c1 +M.sub.s1s h.sub.1s H.sub.c1s)/ (M.sub.s1 h.sub.1 +M.sub.s1s h.sub.1s) (Equation 5)
(in the foregoing relief type MSR, H.sub.c1 &lt;H.sub.CA &lt;H.sub.c1s) where M.sub.s1s, M.sub.c1s and h.sub.1s respectively represent the magnetization, coercive force and thickness of the reproduction sub-layer 31.
In the next place, the extinction type MSR will be explained with reference to FIG. 5. FIG. 5A is a schematic top plan showing a record pattern on a magneto-optical recording medium 10, and FIG. 5B is a schematic sectional view showing a magnetization mode thereof. In FIGS. 5A and 5B, component parts corresponding to those in FIGS. 3A and 3B are denoted by the same reference signs as used in FIGS. 3A and 3B, and the explanation of those parts will not be repeated. This MSR system does not require an initializing magnetic field Hi.
In a reproduction mode of the magneto-optical recording medium 10, the condition expressed by the following Equation 6 is fulfilled in a high-temperature region 14. Thus, the magnetization in the high-temperature region 14, even if in a laser beam spot 5, is aligned in the downward direction by a reproducing magnetic field Hr applied externally. Consequently, record pits 4 in a reproduction layer 11 disappear. Thus, the extinction type MSR system is designed so that reproduction can be performed only for the record pits 4 present in a low-temperature region 15 located in the beam spot 5, thereby offering an improved resolution. EQU Hr&gt;H.sub.c1 +.sigma..sub.w2 /2M.sub.s1 h.sub.1 (Equation 6)
In this case, however, conditions such as the coercive force of a recording layer 13 are set so that even after the extinction (disappearance) of the record pits 4 in the reproduction layer 11, the record pits 4 in the recording layer 13 are left as latent record pits 41. It is thereby ensured that, at room temperature, the magnetization of the recording layer 13, namely, the information pits 4 in the layer 13 are transferred to the reproduction layer 11 and held in the reproducible state.
According to the relief type and extinction type MSR systems described above, the record pits located in a part of the area of the reproducing laser beam spot are reproduced, so as to attain an enhanced resolution in reproduction.
Furthermore, the above relief type and extinction type MSR systems may be used in combination. In that case, a magneto-optical recording medium 10 located under a beam spot 5 is provided with a temperature distribution in which temperature becomes lower from the forward side toward the backward side with respect to the moving direction of the recording medium relative to the beam spot 5, resulting in the formation of a high-temperature region 14, an intermediate-temperature region 16 and a low-temperature region 15 in the area of the beam spot 5, as shown in FIG. 6. The high-temperature region 14 is made to have the function of the extinction type as explained above with reference to FIG. 5, while the intermediate-temperature region 16 and the low temperature region 15 are made to function respectively as the high-temperature region 14 and low-temperature region 15 as explained above with reference to FIG. 1.
According to the MSR system employing both the relief type and the extinction type in combination, the record pit 4 to be developed in relief in the reproduction layer 11, represented by hatching in FIG. 6, can be present only in the limited, intermediate-temperature region 16 defined between the high-temperature region 14 and the low-temperature region 15. A higher resolution can be thereby achieved.
It is thus possible, according to the MSR systems, to achieve superhigh-resolution reproduction without any restrictions imposed by the wavelength .lambda. of the laser beam or the numerical aperture N.A. of the objective lens.
Accordingly, the MSR systems enable a wavelength selection for the reading light to be made taking into account the magneto-optical effect, the temperature rise due to light absorption, the sensitivity of a light detector and the like, without adhering to the use of a shorter wavelength.
In other words, a semiconductor laser light with a comparatively long wavelength (780 nm), for example, can be used as the reading light, to obtain a high reproduction resolution.